package org.colin.Tools;

import lombok.extern.slf4j.Slf4j;

/**
 * @desc snowflake 算法是 twitter 开源的分布式 id 生成算法
 * @author wujiangbo
 * @date 2020年1月17日 上午9:56:48
 */
@Slf4j
public class IdWorker {

	private long workerId;

	private long datacenterId;

	private long sequence;

	private long twepoch = 1288834974657L;

	private long workerIdBits = 5L;

	private long datacenterIdBits = 5L;

	// 这个是二进制运算，就是 5 bit最多只能有31个数字，也就是说机器id最多只能是32以内
	private long maxWorkerId = -1L ^ (-1L << workerIdBits);

	// 这个是一个意思，就是 5 bit最多只能有31个数字，机房id最多只能是32以内
	private long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);

	private long sequenceBits = 12L;

	private long workerIdShift = sequenceBits;

	private long datacenterIdShift = sequenceBits + workerIdBits;

	private long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;

	private long sequenceMask = -1L ^ (-1L << sequenceBits);

	private long lastTimestamp = -1L;

	/**
	 * @param workerId     机器id
	 * @param datacenterId 机房id
	 * @param sequence     序号
	 */
	public IdWorker(long workerId, long datacenterId, long sequence) {
		// sanity check for workerId
		// 这儿不就检查了一下，要求就是你传递进来的机房id和机器id不能超过32，不能小于0
		if (workerId > maxWorkerId || workerId < 0) {
			throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
		}
		if (datacenterId > maxDatacenterId || datacenterId < 0) {
			throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
		}
		log.debug("worker starting. timestamp left shift {}, datacenter id bits {}, worker id bits {}, sequence bits {}, workerid {}", timestampLeftShift, datacenterIdBits, workerIdBits, sequenceBits, workerId);
		this.workerId = workerId;
		this.datacenterId = datacenterId;
		this.sequence = sequence;
	}

	public long getWorkerId() {
		return workerId;
	}

	public long getDatacenterId() {
		return datacenterId;
	}

	public long getTimestamp() {
		return System.currentTimeMillis();
	}

	public synchronized long nextId() {
		// 这儿就是获取当前时间戳，单位是毫秒
		long timestamp = timeGen();

		if (timestamp < lastTimestamp) {
			log.debug("clock is moving backwards.  Rejecting requests until {}", lastTimestamp);
			throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
		}

		if (lastTimestamp == timestamp) {
			// 这个意思是说一个毫秒内最多只能有4096个数字
			// 无论你传递多少进来，这个位运算保证始终就是在4096这个范围内，避免你自己传递个sequence超过了4096这个范围
			sequence = (sequence + 1) & sequenceMask;
			if (sequence == 0) {
				timestamp = tilNextMillis(lastTimestamp);
			}
		} else {
			sequence = 0;
		}

		// 这儿记录一下最近一次生成id的时间戳，单位是毫秒
		lastTimestamp = timestamp;

		// 这儿就是将时间戳左移，放到 41 bit那儿；
		// 将机房 id左移放到 5 bit那儿；
		// 将机器id左移放到5 bit那儿；将序号放最后12 bit；
		// 最后拼接起来成一个 64 bit的二进制数字，转换成 10 进制就是个 long 型
		return ((timestamp - twepoch) << timestampLeftShift) | (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;
	}

	private long tilNextMillis(long lastTimestamp) {
		long timestamp = timeGen();
		while (timestamp <= lastTimestamp) {
			timestamp = timeGen();
		}
		return timestamp;
	}

	private long timeGen() {
		return System.currentTimeMillis();
	}

	// ---------------测试---------------
	public static void main(String[] args) {
		IdWorker worker1 = new IdWorker(1, 1, 1);
		for (int i = 0; i < 5; i++) {
			System.out.println(worker1.nextId());
		}
		System.out.println("*****************************************************************");
		IdWorker worker2 = new IdWorker(2, 2, 1);
		for (int i = 0; i < 5; i++) {
			System.out.println(worker2.nextId());
		}
	}
}
